one hundredth is 0.01
one hundred is 100
a hundredth means that it takes 100 of them to equal 1
rounding would be to "round off" additional digits.
For instance if you wanted to round the number
1.9347 to the nearest hundredth,
you would be wanting to end up with a number with only two decimal places. To round, you typically round up to the nearest digit if the next digit is 5 or more and you generally round down to the nearest digit if the next digit is 0-4. So this example rounds down to 1.93, rounded to the nearest hundredth.
Estimation is more of a way to get the answer without really doing the math. For instance:
If you want to divide 483466554 by 63 and you could not use a calculator but had to choose from 4 answers:
a) 4567
b) 54346
c) 7674072
d) 487475743
You might make the estimation to this difficult division by instead dividing the original number by 100 since 63 is closest to 100. Dividing the number by 100 is easy, the decimal place just moves 2 places to the left to give you:
4834665.54 (round off the decimals)
4834666
Which you notice is a 7- digit number. Dividing by 63 instead of 100 should yield a larger number, but not too much larger. Therefore you pick the only 7-digit number among the answers and choose the correct answer, c.